Let your parents take a tape measure when they go shopping, this is the only way to be sure. The brochure may say 60 x 80 or what ever but the dealer may have ordered a different size to make the bedroom appear more spacious. I totally get their point, I'm 6'3" and a 74" mattress would be an insult to my body. Having said that we had a 5th wheel once with a short queen mattress and put a 6" piece of foam between the mattress and the wall. It may also be possible to fore go the rv mattress completely and go with a residential one.
2013 RAM 3500 Dually Longhorn
2014 Trilogy 3650RL, W/D, Onan 5500, 8000# Axles, IS, 960 Watt Solar, 6x6 Volt Battery Bank, Freedom SW 3012 Inv/Charger